The details of the process of phototransduction are not known, but probably include a number of events intervening between the initial photon capture by the visual pigment, the subsequent photoconversion of that pigment, and the changes in membrane permeability which finally result in the production of the receptor potential. This process will be more completely understood when the membrane mechanisms which mediate ion permeation are known. One approach to the understanding of these membrane mechanisms is to systematically test a variety of models to determine which of them best describe(s) the behavior of the photoreceptor membrane. This will be done in Balanus eburneus by comparing the electrical behavior of the photoreceptor to that predicted by a number of continuum membrane models. These models have grown out of the Nernst-Planck flux equations and generally require fewer assumptions about the detailed structure of the membrane than other types of models (e.g., those based on transition theory) while allowing one to distinguish various mechanisms of ion permeation, i.e., whether ion permeation is mediated by fixed sites (in pores) or mobile sites (carriers) and whether the active membrane sites are neutral (polar) or electrically-charged moieties. Since the light-induced current is known to be carried by Na ion and Ca ions, the mechanisms of alkali-metal and alkaline-earth cation permeation are of particular interest. These two groups of cations will be separately studied after suitable measures have been taken to exclude or quantitate any anion contribution to the light-induced membrane potential or conductance change. Ca ions is known to suppress the light-induced current, and so the effect of alkaline-earth cations on alkali-metal cation permeation will be evaluated to determine the nature of this interaction. The principal experimental techniques will be voltage clamp, ion-substitution, intracellular ion measurement (using ion-selective electrodes) and ion-dilution. The large (100 micron m) photoreceptors of Balanus are well-suited to and quite tolerant of such procedures.